This invention relates to a method of joining two adjacent portions of bone, for example, when replacing a portion of the cranial vault removed during a craniotomy.
In most neurosurgical and cranial operations, it is necessary to open a large access hole in the skull by forming a segment of the skull, called a bone flap, which is then bent out of the way or broken out from the surrounding skull. To form the bone flap, several holes are typically drilled through the skull, commonly referred to as burr holes. The burr holes are then connected by osteotomy cuts, for example using a Gigli flexible saw which is passed internally between the burr holes. The saw is then oscillated back and forth to cut the skull along a line connecting adjoining burr holes. The position, number, and size of the burr holes drilled through the skull, as well as the number of connecting osteotomies, is determined by the size, location and geometrical form of the desired bone flap and corresponding access hole. For example, if the bone flap to be removed is a triangular skull cap segment, three holes are preferably drilled at the corner points of the bone flap, connecting osteotomies are made along the sides of the curved triangle delineated by burr holes, resulting in a triangular segment bone flap. The bone flap is subsequently lifted off the underlying dura mater to expose the brain for the further steps of the operation. The bone flap may either be completely removed from the surgical site, or folded back along an uncut edge.
At the end of the procedure, the previously removed bone flap or flaps are repositioned into their original locations, or in different desired locations, relative to the surrounding bone portions. This is typically accomplished in the prior art by drilling small pairs of holes in the surrounding skull bone in several places around the edge of the bone flap. Wire is then carefully threaded through the holes, taking care not to tear the dural tissue covering the brain, then twisted together to secure the edges, the ends tucked into the cut opening so that they do not puncture the skin, and the skin then stitched into place over the skull flap. The procedure is complex and time consuming, and there always is the possibility of injuring the dura either by using the high speed drills that are necessary to form the small holes or by the sharp points of the wire engaging the dura.
Other known methods for providing fixation between adjacent bone portions have included the use of metallic plates of varying configurations which are secured across osteotomies or fracture sites by metallic bone screws. Other devices, such as intramedullary nails or externally fixed pins, have also been used to reduce bone fracture mobility and to improve the relative position of adjacent segments. See for instance U.S. Pat. No. 5,669,912 to Spetzler. The aim of fixation of adjacent bone portions is to immobilize the fracture or osteotomy sites in order to promote localized bone growth in the natural repair of the separation.
A brief survey of prior art methods may be found by looking at patents previously issued on the subject. For instance, U.S. Pat. No. 5,201,737 discloses a flexible plate having a plurality of vanes with holes for receiving bone screws. The plate is placed over a cranial burr hole and adjoining osteotomy lines to provide external fixation of the bone flap to the surrounding cranium. Other external bone plates are shown in U.S. Pat. Nos. 4,651,724; 4,923,471; 5,139,497; 5,372,498; and 5,578,036. All of these plates are designed for external application to fractured bones and require placement of a plurality of screws through the plates and into the bone. Placement of multiple screws through the plates is time consuming, induces additional trauma in drilling the pilot holes for the screws, and may predispose the site to infection.
Other fixation devices are also known, such as the device shown in U.S. Pat. No. 2,511,051 which involves screwing an externally threaded stud into an internally threaded shank. Movement of the stud into the shank is guided by an hexagonal wrench that is inserted through the shank into a countersunk receptacle on the tip of the threaded stud. A similar device is shown in U.S. Pat. No. 5,707,373. These devices have proved cumbersome to use. Further, these devices necessarily require that a portion of the fastener be disposed on both the inner and outer surfaces of the bone, thereby exposing the dura matter of the brain to direct intimate contact with the fastener.
In spite of the use of a variety of fasteners in surgical procedures, improved techniques are still being sought to secure adjacent portions of bone for healing, particularly for securing bone flaps to the surrounding cranium following a craniotomy.
The present invention utilizes an expandable fastener, called a bone lock, to join adjacent portions of bones for surgical recovery. The bone lock includes main body having an expandable section and an expansion driver. Moving the expansion driver from a first position to a second position forces the expandable section to expand, thereby causing the bone lock to engage the nearby bone material.
One preferred embodiment of the bone lock includes a sleeve having an expandable section and a ram that is moveable between a ready position and a deployed position relative to the sleeve. When the ram is moved to the deployed position, the ram acts against the sleeve to expand the sleeve""s expandable section. When expanded in a burr hole, the sleeve is forced against at least one of the bone sections, thereby constraining the relative motion between the bone portions.
Preferably, the sleeve includes a plurality of prongs in the expandable section. These prongs are forced outwardly when the ram is moved to the deployed position. In most embodiments, the prongs include protrusions having bearing surfaces that curve outwardly from the bore of the sleeve. When the sleeve is expanded, bearing surfaces on the protrusion preferably act against the bone to hold the bone lock securely against the bone. Preferably, the bearing surfaces apply both a lateral and a radial force, with the radial force acting to clamp the bone between the prong and a flange on the sleeve. The bone lock may clamp only one of the bone portions with such an embodiment, but it is preferred that the bone lock clamp onto both bone portions.
Further, in some embodiments, the bone lock is designed to be shorter than the surrounding bone is thick. With such embodiments, the bone lock may be used to secure adjacent bone sections without extending into the cranial cavity by instead extending into the medullary layer of the surrounding bone portions.
During the closure portion of a typical craniotomy, the bone flap is appropriately positioned in the craniotomy opening and a bone lock is inserted into at least one of the burr holes formed earlier in the craniotomy. The bone lock described above is expanded by driving the ram down into to sleeve, typically by screwing the ram into the bore of the sleeve. The movement of the ram forces the sleeve to expand, thereby restricting the relative motion of the two bone portions. Ideally, the bone portions are secured both laterally (approximately along the skull surface) and radially (generally normal to the skull surface) by the bone lock. Further, while it is not required, it is preferred that all burr holes be filled by a bone lock, thereby providing multi-point fixation of the bone flap to the surrounding cranial bone.
The use of the present approach allows the bone sections to be joined without the creation of additional holes in the skull or bone flap, particularly small screw holes, thereby saving time and reducing the risk of infection. In addition, the use of the optional shorter bone lock embodiments allows the bond flap to be secured without having portions of the fasteners protruding into the cranial cavity, thereby lessening the risk of injury to nearby soft tissue, such as the brain""s dura matter.